A. Technical Field
The present invention relates generally to an actuator driver in a camera module, and more particularly to methods, systems and devices of employing self-calibrated ringing compensation to improve the autofocus convergence time of the camera module that is based on a voice coil motor (VCM) or other actuator with resonant behavior. The free-oscillation frequency and damping coefficient of the VCM are calibrated, and used to control a sink current that drives the VCM.
B. Background of the Invention
Camera phones have been the fast-growing consumer electronic device in the market, and miniaturized camera modules are included in these phones. Size and cost have always been of the primary importance to these camera modules, while real camera performance is also on high demand. In particular, evolution of the camera modules has been driven by a demand for high resolution. In order to make the best use of the increasing resolution, effective and rapid autofocus (AF) is needed to accompany the increase in pixels. Today, autofocus has become a standard feature in most camera modules, and thereafter, enables more camera features including optical zoom, shutter control and image stabilization.
Autofocus is dependent from the ability of a lens driver to move the lens of a camera module rapidly so as to achieve focus of an image. A typical lens actuator may be based on a digital stepper motor, a piezoelectric motor, a MEMS actuator, or a voice coil motor (VCM). The lens driver varies according to the lens actuator in use. For example, the lens driver generates a voltage to drive the digital stepper or piezoelectric motor, or a current to drive the VCM. The VCM-based lens actuator is used in more than 90% of current camera modules, and therefore, a lens driver is needed to generate a desirable current for rapid positioning and autofocus of the lens in most state-of-the-art camera modules.
The VCM-based lens actuator is analogous in concept and operation to a miniaturized speaker. The lens barrel is wrapped with a coil that is placed in close proximity to a permanent magnet, or group of magnets. A spring maintains the lens barrel at an infinite-focus position (i.e., a park position) that is associated with no power dissipation. When a sink current is injected to the voice coil, a Lorentz force produced by the interaction of the current in the coil and the permanent magnets causes the lens barrel to move to a static position determined by the balance of forces between the Lorentz force and spring return force. FIG. 1A illustrates a typical relationship 100 between the lens displacement and the sink current for a VCM-based lens actuator. The sink current needs to exceed a threshold current to cause any displacement from a park position. Beyond the threshold current, the lens displacement has a linear correlation with the sink current until it reaches a maximum displacement. Despite its current consumption at the focus position, the VCM-based lens actuator is a low-cost solution that is mechanically robust, shock-resistant and exhibits relatively low hysteresis.
A closed-loop autofocus method is employed to move the lens in a VCM-based camera module to its focus position. FIG. 1B illustrates a relationship 150 between an autofocus value (AFV) and the lens position in a closed-loop autofocus method that is based on a standard hill-climbing algorithm. The AFV is associated with the quality of an image, and an exemplary AFV is the contrast of the image. In a coarse scanning step, the sink current increases at a relatively large current step to allow the lens displacement to scan from zero to the maximum displacement rapidly. The AFV is analyzed to indicate the quality of the image, and particularly, an AFV peak is approximately identified. In a subsequent fine scanning step, the lens is moved back to the position associated with the AFV peak at a smaller step, and located to a focus position in a final positioning step. The focus position is close to the position associated with the AFV peak in the coarse scanning step, and may be accurately identified in the fine scanning.
The spring-loaded VCM system is modeled as a damped harmonic oscillator, and therefore, during each lens positioning sub-step in the autofocus process, the lens position is associated with overshooting and oscillation before the lens settles at the target position. FIG. 2 illustrates an exemplary time diagram 200 of a lens position that is associated with a VCM-based lens actuator driven by a step sink current. When the current step is applied to the VCM, the lens barrel moves from its park position, overshoots to a peak location, oscillates at its free-oscillation frequency, and settles at the target lens position. Such an overshooting-oscillating-settling process may take a settling time of up to 100-200 msec (i.e., 5-10 oscillation cycles) to complete each lens positioning sub-step in the closed-loop autofocus. This settling time may not be accommodated to most of the state-of-the-art camera modules that are capable of capturing still images and video frames at a fast autofocus rate.